1. Field of the Invention
The present invention relates to solenoid actuators. More specifically, the present invention is directed to an A.C. power control circuit for direct current solenoid valves.
2. Description of the Prior Art
Alternating current (A.C.) operated solenoid valves have functional characteristics that make them acceptable for long stroke operation. This long stroke capability is a result of low open gap inductance which means low impedence and a high initial current. When the gap closes, the inductance increases to increase the impedence and produce a low holding current. This operation occurs automatically during the seating of the plunger. However, if for any reason the plunger does not seat and the gap remains open, then the solenoid coil current remains high and coil burnout is possible. On the other hand, the direct current (D.C.) operated solenoid requires large power for producing large strokes. However, when the plunger is seated, very little change is power occurs due to the lack of impendence change. However, the D.C. operated solenoid valve is free from the "buzz" problem associated with A.C. operated solenoid valve which requires shading rings to minimize the "buzz". Accordingly, it would be desirable to provide a valve having functional characteristics which incorporate the desirable features of the A.C. and D.C. operated solenoids, i.e., operation off an A.C. line without shading rings and no "buzz", large stroke capability and high current pull-in and low current hold in a package which must be cost effective and fit within conventional valve housings. One prior art approach was to use a diode in series with the solenoid coil to permit D.C. "half" wave operation from the A.C. line. In order to obtain the long stroke forces for large flow valves, the average power required would be quite high and without the A.C. type impedence change the holding power would be the same as the pull-in power. A subsequent device used a "full" wave bridge rectifier circuit. This had the advantage of eliminating the "buzz" and would require less input power because the average magnetic flux per cycle is higher. The major limitation with this approach is that the line common and the solenoid common cannot be the same which presents valve packaging problems because the valve body could not be connected to a common reference. Accordingly, it would be desirable to provide an A.C. power control circuit for operating a D.C. valve while eliminating the aforesaid limitations of the prior art circuits.